The present invention relates to a superconducting magnet apparatus that is suitable for use in a magnetic resonance imaging system (hereunder referred to as xe2x80x9cMRI systemxe2x80x9d) and, more particularly, to a superconducting magnet apparatus that has a large opening to thereby prevent a subject from feeling claustrophobic and to thereby allow an operator to have easy access to a subject.
FIG. 7 illustrates an example of a conventional superconducting magnet apparatus for use in MRI system. This example is a superconducting magnet apparatus of the horizontal magnetic field type. This superconducting magnet apparatus is composed of small-diameter main coils 13, 14, 15, 16, 17 and 18 and large-diameter shield coils 19 and 20 and is adapted to produce a horizontal (namely, Z-axis direction) magnetic field. In this example, the main coils 13 to 18 are placed to produce a magnetic field along the center axis 22 of a magnet, while the shield coils 19 and 20 are placed to shield magnetic field leakage to the surroundings thereof. With such a configuration of the magnet, a uniform magnetic field region 21 of magnetic homogeneity of about 10 ppm or less is formed in a magnetic field space. Magnetic resonance imaging pictures are taken in this uniform magnetic filed 21.
These coils are generally made by using superconducting wires, and thus need cooling to a predetermined temperature (for example, liquid-helium temperature (namely, 4.2 K) in the case of alloy superconductors; and liquid-nitrogen temperature (namely, 77 K) in the case of oxide superconductors). The coils are, therefore, held in a cooling vessel consisting of a vacuum enclosure, a thermal shield and a coolant container (containing liquid helium or the like). In the case of the example of FIG. 7, the main coils 13 to 18 and the shield coils 19 and 20 are placed in a coolant container 11, which contains coolant 12, such as liquid helium, for superconductivity by supported by means of supporting elements (not shown). Further, the coolant container 11 is held in a vacuum enclosure 10.
Moreover, to hold each of the coils at a low temperature, the thermal shield is maintained at a constant temperature by using a refrigerator (not shown) or the evaporation of coolant 12 for superconductivity is reduced. Recently, the performance of the refrigerator has been increased, so that the superconductor coils are sometimes cooled directly by the refrigerator without using the coolant container 11.
However, in the case of the superconducting magnet apparatus illustrated in FIG. 7, an opening, in which a subject is accommodated and images of the subject are taken, is narrow and moreover, a measuring space is surrounded, so that subjects sometimes feel claustrophobic. Thus, occasionally, subjects refuse to enter the opening of the apparatus for examination. Furthermore, it is difficult for an operator to get access to a subject from outside the superconducting magnet apparatus.
FIG. 8 illustrates another example of a conventional superconducting magnet apparatus for use in MRI system. This example is an open superconducting magnet apparatus of the horizontal magnetic field type. This example of the conventional superconducting magnet apparatus has been disclosed in the U.S. Pat. No. 5,410,287 and remedies the drawbacks of the aforementioned example of the conventional superconducting magnet apparatus of FIG. 7 in that the measuring space causes a subject to feel claustrophobic and in that there is the difficulty in getting access to a subject by an operator. FIG. 8(a) shows a sectional view of this example. FIG. 8(b) shows an external view thereof. As shown in FIG. 8(a), a set of three coils 23A, 24A and 25A and another set of three coils 23B, 24B and 25B are spaced apart from each other by a predetermined distance in such a manner as to be coaxial with the center axis 22 of a magnet. Further, a uniform magnetic field region 21 is generated at the halfway position between the sets of the coils. Coils of each of the sets are supported by supporting elements (not shown) and are directly cooled by a refrigerator. All of the coils of each of the sets are surrounded with thermal shields 9A and 9B that are held in vacuum enclosures 10A and 10B, respectively.
Coils 23A, 23B, 24A and 24B are main coils, through which electric currents flow in a same direction. Coils 25A and 25B are auxiliary coils, through which electric currents flow in a direction opposite to the direction of the current flow in the main coils. In the magnet having this configuration, the main coils 23A, 23B, 24A and 24B produce a magnetic field along the center axis 22. Further, the auxiliary coils 25A and 25B enhance the magnetic homogeneity of the uniform, magnetic field region 21. Incidentally, this magnet does not use shield coils. However, a room, in which the superconducting magnet apparatus is installed, is magnetically shielded.
Further, as illustrated in FIG. 8(b), the vacuum enclosures 10A and 10B facing each other in the lateral direction are shaped like doughnuts and are supported by two supporting posts 26 interposed therebetween. Thus, there is provided an open space between the vacuum enclosures 10A and 10B. A subject is inserted into the uniform magnetic field region 21 along the center axis 22, which is illustrated in FIG. 8(a), through the central bores of the vacuum enclosures 10A and 10B. Then, images of the subject are taken there.
In accordance with such a configuration, outward side surfaces of the uniform magnetic field region 21 serving as an imaging region are opened. Thus, a subject can avoid feeling claustrophobic. Moreover, an operator can easily get access to the subject from a side of the apparatus and further can use the images displayed on the screen of a monitor during an operation.
However, in the case of the superconducting magnet apparatus illustrated in FIG. 8, each of the sets of coils 23A, 24A and 25A and coils 23B, 24B and 25B and the vacuum enclosures 10A and 10B is shaped like a doughnut. Thus, a space between the doughnut-like vacuum enclosures 10A and 10B facing each other is not used as a region used for performing improvement in magnetic homogeneity. Therefore, it has been difficult to obtain favorable magnetic homogeneity over a large space. Further, magnetic fluxes generated by the superconducting coils return through the external space of the superconducting magnet apparatus, so that a leakage magnetic field becomes large. Thus, a large area is needed for installing the superconducting magnet apparatus. Alternatively, strong magnetic shielding should be performed.
FIG. 9 illustrates a third example of a conventional superconducting magnet apparatus for use in MRI system. This example is a superconducting magnet apparatus of the vertical magnetic field type. This example of the conventional superconducting magnet apparatus has been disclosed in the U.S. Pat. No. 5,194,810. This magnet enhances the magnetic homogeneity of a uniform magnetic field region 21 by generating a magnetic field by the use of two sets of superconducting coils 31 and 31, the respective sets of which are placed vertically in such a way as to face each other, and by providing iron shimming means 32 on the inner surfaces of the aforesaid superconducting coils 31 and 31 so as to obtain favorable magnetic field homogeneity. Moreover, this magnet has a structure in that upper and lower magnetic-field generating sources are mechanically supported by iron yokes 33, 33, . . . that further serve as return paths for magnetic fields generated by the upper and lower superconducting coils 31 and 31.
In the case of this example of the conventional superconducting magnet apparatus, the uniform magnetic field region 21 is opened in all directions, a subject can avoid feeling claustrophobic. Moreover, an operator can easily get access to the subject. Further, magnetic field leakage can be reduced because of the fact that the return path of the magnetic flux is composed of the aforementioned iron yokes 33, 33, . . . and upper and lower iron plates 34 and 34.
However, in the case of the superconducting magnet apparatus illustrated in FIG. 9, there are caused the problems that the iron yokes 33 and the iron plates 34 are used as above described, so that the entire magnet becomes heavy and that thus, when installing the superconducting magnet apparatus, an installation floor needs to be strengthened. Further, because the saturation magnetic flux density of iron is approximately 2 Tesla or so, there is a restraint on the magnet in that the magnetic field strength cannot be increased to a high value. Furthermore, because of the hysteresis characteristics of iron with respect to the magnetic field, a magnetic field generated by a gradient magnetic field coil affects the magnetic field distribution. This may hinder high-precision signal measurement.
As above described, there have been no conventional superconducting magnet apparatuses wherein openings, in which a subject is inserted, are enlarged so as to prevent the subject from feeling claustrophobic, wherein magnetic field leakage is low, and wherein the weight of the entire apparatus is reduced by avoiding using much iron. Moreover, in the case of the conventional apparatuses, it is difficult to realize a large uniform magnetic-field region having high-magnetic-strength.
It is, accordingly, an object of the present invention to provide a superconducting magnet apparatus that deals with such problems of the conventional superconducting magnet apparatuses, that enlarges an opening, which accommodates a subject, so as to prevent the subject from feeling claustrophobic, that has low magnetic field leakage, that avoids using much iron so as to reduce the weight thereof, and that can realize a large high magnetic field strength uniform magnetic field region.
To achieve the foregoing object of the present invention, in accordance with the present invention, there is provided a superconducting magnet apparatus comprising: magnetic field generating sources, which are made of substances having superconducting properties and are operative to feed electric current for generating a uniform magnetic field, whose direction is a first direction, in a finite region; cooling means for cooling the aforesaid magnetic field generating sources to a temperature, at which the substances exhibit the superconducting properties, and for maintaining the aforesaid magnetic field generating sources at the temperature; and supporting means for supporting the magnetic field generating sources. In this apparatus, the magnetic field generating sources are placed equidistantly in such a way as to face each other across the uniform magnetic field region along the first direction and are composed of two sets of magnetic field generating device groups for feeding electric current in a coaxial direction with the first direction being made the center axis. In this apparatus, each of the sets of magnetic field generating device groups is composed of: one or more first magnetic field generating devices for feeding electric current which flows in a second direction along the circumference of a circle, whose center axis extends in the aforesaid first direction, so as to generate a main component of the uniform magnetic field; one or more second magnetic field generating devices for feeding electric current which flows in a direction opposite to the second direction, so as to reduce a magnetic field generated outside the magnetic field generating sources; and one or more third magnetic field generating devices for feeding electric current which flows in a direction that is same as or opposite to the second direction, so as to improve magnetic homogeneity of the uniform magnetic field. The diameter of the first magnetic field generating device is nearly equal to that of the second magnetic field generating device. Further, the diameter of the third magnetic field generating device is less than that of the first magnetic field generating device. An amount of electric current fed to the third magnetic field generating device is less than an amount of electric current fed to the first magnetic field generating device. The distance between the first magnetic field generating devices facing each other across the uniform magnetic field region is shorter than that between the second magnetic field generating devices facing each other across the uniform magnetic field region.
Thereby, an apparatus, which has a large opening and small magnetic field leakage, can be realized. Further, iron is not used in order to suppress the magnetic field leakage. Thus, the weight of the apparatus can be reduced. Moreover, there is not caused a magnetic flux saturation which would be a problem caused if using iron. Therefore, even if the magnetic field strength becomes high, favorable magnetic homogeneity can be attained over the large uniform magnetic field region.
Furthermore, with the apparatus of the present invention, the diameter of the aforesaid second magnetic field generating device may be set as being larger than that of the first magnetic field generating device. Thereby, the efficiency of the second magnetic field generating device can be increased. Consequently, the magnetic field leakage to the exterior of the apparatus can be more effectively reduced.
Additionally, with the apparatus of the present invention, the second magnetic field generating devices each consists of magnetic field generating elements having a plurality of different diameters. The magnetic field generating elements are provided in such a manner that the diameter of the magnetic field generating elements corresponding to each other is increased in proportion to the distance between these magnetic field generating elements corresponding to each other and facing each other across a uniform magnetic field generating region. Thereby, the electromagnetic force acting on the second magnetic field generating elements obtained by dividing the second magnetic field generating device can be lowered. Moreover, the conditions for manufacturing the superconducting magnet apparatus can be moderated.
Besides, with the apparatus of the present invention, the first magnetic field generating devices each consists of magnetic field generating elements having a plurality of different diameters. These magnetic field generating elements are provided in such a manner that the diameter of these magnetic field generating elements corresponding to each other is increased in proportion to the distance between these magnetic field generating elements corresponding to each other and facing each other across the uniform magnetic field generating region. Thereby, the electromagnetic force acting on the first magnetic field generating elements obtained by dividing the first magnetic field generating device can be reduced.
Further, with the apparatus of the present invention, the distance between the first magnetic field generating devices facing each other across the uniform magnetic field generating region may be set as being less than the distance between the second magnetic field generating devices facing each other across the uniform magnetic field generating region and further may be set as being less than the distance between the third magnetic field generating devices facing each other across the uniform magnetic field generating region. Thereby, the magnetic field strength of the magnetic field produced in the uniform magnetic field region can be increased without increasing the magnetomotive force of the first magnetic field generating device.
Moreover, with the apparatus of the present invention, the cooling means has vacuum enclosures for containing the magnetic field generating sources. Furthermore, an outer circumferential portion of each of the vacuum enclosures protrudes to a side at which the uniform magnetic generating region exists.
In addition, with the apparatus of the present invention, each of the magnetic field generating sources is constituted by a coil obtained by winding a wire made of a substance having a superconducting property.